Partisan Gerrymandering and the Efficiency Gap

The usual legal story about partisan gerrymandering is relentlessly pessimistic. The courts did not even recognize the cause of action until the 1980s; they have never struck down a district plan on this basis; and four sitting justices want to vacate the field altogether. The Supreme Court’s most recent gerrymandering decision, however, is the most encouraging development in this area in a generation. Several justices expressed interest in the concept of partisan symmetry—the idea that a plan should treat the major parties symmetrically in terms of the conversion of votes to seats—and suggested that it could be shaped into a legal test.

In this Article, we take the justices at their word. First, we introduce a new measure of partisan symmetry: the efficiency gap. It represents the difference between the parties’ respective wasted votes in an election, divided by the total number of votes cast. It captures, in a single tidy number, all of the packing and cracking decisions that go into a district plan. It also is superior to the metric of gerrymandering, partisan bias, that litigants and scholars have used until now. Partisan bias can be calculated only by shifting votes to simulate a hypothetical tied election. The efficiency gap eliminates the need for such counterfactual analysis.

Second, we compute the efficiency gap for congressional and state house plans between 1972 and 2012. Over this period as a whole, the typical plan was fairly balanced and neither party enjoyed a systematic advantage. But in recent years—and peaking in the 2012 election—plans have exhibited steadily larger and more pro-Republican gaps. In fact, the plans in effect today are the most extreme gerrymanders in modern history. And what is more, several are likely to remain extreme for the remainder of the decade, as indicated by our sensitivity testing.

Finally, we explain how the efficiency gap could be converted into doctrine. We propose setting thresholds above which plans would be presumptively unconstitutional: two seats for congressional plans and 8 percent for state house plans, but only if the plans probably will stay unbalanced for the remainder of the cycle. Plans with gaps above these thresholds would be unlawful unless states could show that the gaps either resulted from the consistent application of legitimate policies or were inevitable due to the states’ political geography. This approach would neatly slice the Gordian knot the Court has tied for itself, explicitly replying to the Court’s “unanswerable question” of “[h]ow much political . . . effect is too much.”